An electronic article surveillance system 20 is shown in schematic terms in FIG. 1. The system 20 is typically provided at the exit of a retail store to detect the presence of a marker 22 in an interrogation zone 24 defined between antenna pedestals 26 and 28. When the system 20 detects the marker 22, the system 20 actuates an alarm of some kind to indicate that an article (not shown) to which the marker 22 is secured is being removed from the store without authorization.
Customarily, each of the antenna pedestals 26 and 28 is generally planar and includes one or more loop antennas. Signal generating circuitry 30 is connected to the antenna or antennas in pedestal 26 to drive the antennas in pedestal 26 to generate an interrogation signal in the interrogation zone. Also, receiver circuitry 32 is connected to the antenna or antennas in the pedestal 28 to receive and analyze signals picked up from the interrogation zone by the antennas in the pedestal 28.
For purposes of further discussion, a coordinate system 34, consisting of X, Y and Z axes, mutually orthogonal to each other, is shown in FIG. 1. The antenna pedestals 26 and 28 are usually arranged in parallel to each other, and for the purposes of this and further discussion, it should be understood that the respective planes of the pedestals 26 and 28 are parallel to the plane defined by the Z and X axes. The Z axis is presented as being a vertical axis, and the X axis is a horizontal axis extending in the direction of a path of travel through the interrogation zone 24, i.e., parallel to the planes of the pedestals 26 and 28. The Y axis is also horizontal, but in a direction perpendicular to the X axis. For some purposes, the X direction will be referred to as the "horizontal direction", the Z direction will be referred to as the "vertical direction", and the Y direction will be referred to as the "lateral direction".
The marker 22 typically includes a coil or other planar element that receives the interrogation signal generated through the antenna pedestal 26 and retransmits the signal, in some fashion, as a marker signal to be detected through the antenna pedestal 28. The amplitude of the marker signal is, in general, dependent on the orientation of the plane of the receiving element in the marker 22. As a practical matter, the orientation of the plane of the receiving element has three degrees of freedom, but the response of the marker can be analyzed in terms of components corresponding to three orthogonal plane orientations. These will be referred to as a "horizontal orientation", corresponding to the plane defined by the X and Y axes, a "vertical orientation", corresponding to the plane defined by the Z and X axes, and a "lateral orientation", corresponding to the plane defined by the Z and Y axes.
For markers used in magnetomechanical EAS systems, the marker responds to flux that is co-planar with the marker, but for markers that include a coil, the marker responds to flux that is orthogonal to the plane of the coil. Subsequent discussions herein will be based on the assumption that a magnetomechanical marker is in use.
It is generally an objective in an EAS system that the system reliably detect any marker in the interrogation zone, regardless of position in the zone or orientation of the marker. At the same time, it is highly desirable that the system not produce false alarms either by interpreting a signal generated by a non-marker object in or out of the interrogation zone as coming from a marker, or by stimulating markers not in the interrogation zone to generate signals at a level sufficiently high to be detectable by the receiver circuitry.
One significant obstacle to achieving these objectives is the uneven interrogation field distribution commonly provided by antennas used for generating the interrogation signal. As a result of the uneven field distribution, the interrogation field may be strong enough at some or most locations in the interrogation zone to provide for detection of a marker, while not being strong enough at other locations to provide for detection. The locations in which the field is too weak to provide for detection are sometimes referred as "null" areas or "holes".
This problem is aggravated by the fact that the strength of the signal generated by the marker is dependent on the orientation of the marker. Accordingly, a marker at a given location in the zone and oriented in a first manner may be readily detectable, while if the marker is at the same location but oriented in a different manner, the marker would not be detected.
One approach that has been contemplated for overcoming this problem is simply to increase the overall strength of the interrogation field, i.e., by increasing the level of the signal used to generate the interrogating antenna.
Aside from the increased power consumption requirements resulting from this approach, there are often regulatory or other practical constraints on the peak signal level that can be generated. For example, increasing the peak field strength could lead to increased false alarms from either or both of non-marker objects in the interrogation zone and markers located outside of the intended interrogation zone.
Further, in addition to the usual desire to confine the interrogation field to the intended zone, it may be a regulatory requirement, or desirable for other reasons, to provide far-field cancellation of the interrogation signal. This requirement places additional constraints on the design of the antenna used for generating the interrogation signal.